Sample grunt configuration (for sassified underscores.me theme)

Gruntfile.js

module.exports = function(grunt) {
    const sass = require('node-sass');
    grunt.initConfig({
        pkg: grunt.file.readJSON('package.json'),
        sass: {
            options: {
                sourceMap: false,
                implementation: sass
            },
            dist: {
                files: {
                    'wp-content/themes/example-theme/style.css': 'wp-content/themes/example-theme/sass/style.scss'
                }
            }
        },
        postcss: {
            options: {
                processors: [
                    require('autoprefixer')({
                        browsers: ['last 2 versions']
                    })
                ]
            },
            dist: {
                src: 'wp-content/themes/example-theme/style.css',
                dest: 'wp-content/themes/example-theme/style.css'
            }
        },
        watch: {
            grunt: {
                options: {
                    reload: true
                },
                files: ['Gruntfile.js']
            },
            sass: {
                files: ['wp-content/themes/example-theme/**/*.scss'],
                tasks: ['sass', 'postcss']
            }
        },
        browserSync: {
            dev: {
                bsFiles: {
                    src: [
                        'wp-content/themes/example-theme/style.css',
                        'wp-content/themes/example-theme/**/*.php'
                    ]
                },
                options: {
                    watchTask: true,
                    proxy: 'localhost/example-wp'
                }
            }
        }
    });

    //register tasks
    grunt.loadNpmTasks('grunt-sass');
    grunt.loadNpmTasks('grunt-contrib-watch');
    grunt.loadNpmTasks('grunt-browser-sync');
    grunt.loadNpmTasks('grunt-postcss');
    
    grunt.registerTask('default', ['sass', 'postcss', 'browserSync', 'watch']);
};

package.json

{
  "name": "example.com",
  "version": "1.0.0",
  "description": "Example grunt package",
  "repository": {
    "type": "git",
    "url": "git+https://example.com/example.git"
  },
  "scripts": {
    "build": "grunt"
  },
  "author": "Me and myself",
  "license": "UNLICENSED",
  "homepage": "https://example.com",
  "devDependencies": {
    "autoprefixer": "^8.2.0",
    "grunt": "^1.0.3",
    "grunt-browser-sync": "^2.2.0",
    "grunt-cli": "^1.3.2",
    "grunt-contrib-watch": "^1.1.0",
    "grunt-postcss": "^0.9.0",
    "grunt-sass": "^3.0.2",
    "node-sass": "^4.10.0"
  }
}